1. The Field of the Invention.
The present invention relates to methods and apparatus for performing extrapulmonary blood gas exchange wherein blood receives oxygen and releases carbon dioxide. More particularly, the present invention relates to systems used to deliver ventilatory gases to extrapulmonary blood gas exchange devices.
2. The Prior Art
Thousands of patients in hospitals suffer from inadequate blood gas exchange, which includes both inadequate blood oxygenation and inadequate removal of carbon dioxide (CO.sub.2). These conditions are commonly caused by varying degrees of respiratory inadequacy usually associated with acute lung illnesses such as pneumonitis, atelectasis, fluid in the lung, or obstruction of pulmonary ventilation. Various heart and circulatory aliments such as heart disease and shock can adversely affect the flow of blood and thereby also reduce the rate of blood gas exchange.
Currently the most widely used methods of treating these types of blood gas exchange inadequacies involve increasing the flow of oxygen through the lungs by either increasing the oxygen concentration of the inspired gases or by mechanically ventilating the lungs. Both methods result in placing further strain on the lungs, which may be diseased and unable to function at full capacity. In order to allow diseased or injured organs to heal it is generally best to allow these organs a period of rest followed by a gradual increase in activity.
Various devices have been developed which are capable, at least for a limited period of time, of taking over the gas exchange function of the lungs. Many blood oxygenators are in common use and are employed most frequently during heart surgery. Such commonly available devices are capable of providing blood oxygenation and carbon dioxide removal sufficient to carry the patient through the surgical procedure but are not intended to provide pulmonary support for more than the hours required to perform the surgery. These oxygenators include devices which bubble oxygen into the blood as the blood flows through the device. This is usually followed by a portion of the device which removes the bubbles in the blood to make it acceptable for reintroduction into the patient.
Another group of blood oxygenators employ gas permeable membranes. These devices take many different shapes and configurations; however, the basic concept of operation is the same in all of these devices. Blood flows on one side of the gas permeable membranes while a ventilatory gas, i.e., oxygen, flows on the other side of the membrane. As the blood flows through the device the oxygen diffuses, on a molecular level, across the gas permeable membrane and enters the blood. Likewise, carbon dioxide present in the blood diffuses across the gas permeable membrane and enters the gas phase.
Of the available blood oxygenators, those incorporating gas permeable membranes may be best used in long term applications (e.g., one to seven days) as a pulmonary assist device for a patient suffering from acute respiratory failure. In the case of cardiopulmonary bypass where all of the patient's gas exchange needs must be supplied by the oxygenator, constant attention by a trained perfusionist is necessary to guard the welfare of the patient.
The use of a blood oxygenator as a pulmonary assist device also requires constant vigilance if maximum blood gas transfer is to take place. As will be appreciated, the condition of the patient may change from hour to hour, or minute to minute. Such changes often require a change in the operation of a blood oxygenator in order to maintain efficient blood gas transfer. Significantly, some changes in a patient's condition can lead to serious consequences if corresponding changes are not made in the oxygenator's operation. For example, "outgassing," or the forcing of undissolved gas through the membrane into the blood as bubbles where they can form gas emboli, may occur if the blood phase pressure drops dramatically and the gas phase pressure at the permeable membrane is allowed to remain above the blood phase pressure.
In general, perfusionists are able to satisfactorily control the operational characteristics of blood oxygenators using manual control techniques over the duration of a surgical procedure lasting many hours with an acceptably low incidence of operator and equipment related accidents. It will, however, be appreciated that as the length of time a patient is undergoing pulmonary support increases to several days, the likelihood of operator error greatly increases. Furthermore, many of the parameters which must be considered in order to maximize patient welfare are not easily ascertainable using manual techniques. All of these considerations must be addressed when planning to use a pulmonary assist device on a long term basis.
In view of the foregoing, it would be an advance in the art to provide a blood oxygenator gas delivery system which is safer to use than previous available ventilatory gas delivery systems and which can be used with a membrane oxygenator to more efficiently transfer oxygen to, and carbon dioxide from, the blood. It would also be an advance in the art to provide a blood oxygenator gas delivery system wherein the gas phase is always maintained at a low enough pressure to ensure that formation of gas emboli in the blood does not occur and wherein the flow of the gas is precisely and automatically controlled. It would be yet another advance in the art to provide a blood oxygenator gas delivery system which may be easily set up and operated for long periods of time without constant attention from a technician.